1. Field of the Invention
The present invention relates to an optical information reproducing apparatus and an optical information reproducing method and is applicable to a reproducing method and a reproducing apparatus of disks such as a magneto-optical disk (MO), a compact disk (CD, CD-ROM) and a digital video disk (DVD).
2. Description of the Related Art
For example, a conventional recording apparatus for the compact disk being an optical information recording medium of this kind processes data to be recorded and then makes an EFM (Eight-to-Fourteen Modulation) on the data, thereby causing a pit sequence having periods ranging from 3T to 11T with respect to a predetermined basic period T to be formed. Thus, audio data or the like to be recorded.
Correspondingly, a compact disk player irradiates the compact disk with a laser beam and receives the returned light, thereby generating a reproduced signal which varies in the signal level dependent on an amount of the returned light and then producing a binary signal from the reproduced signal by a predetermined slice level.
Further, this binary signal drives a PLL circuit to produce a reproduced clock, by which the binary signal is sequentially latched. This reproduces data with the periods from 3T to 11T corresponding to the pit sequence framed on the compact disk to be produced.
The compact disk player decodes the thus reproduced data by the data processing corresponding to the data processing during recording and reproduces audio data or the like recorded on the compact disk.
In this context, every optical disk has different characteristics of the degree of modulation, the reflective factor and the like. For this reason, in the optical disk reproducing apparatus, there is provided a laser power control circuit having a function which prevents the deterioration of reproduced signal level by controlling the level of reproduced RF signal to be kept constant.
FIG. 5 shows a block diagram of the laser power control circuit of a conventional optical disk device. The optical disk device to which the laser power control circuit is applied comprises an auto power control (APC) circuit 1, an invertor 6, a laser 7 emitting a laser light, a disk 8 irradiated with the laser light, a detector 9 detecting the reflected light of the laser light irradiating the disk 8, a RF amplifier 10 amplifying the reproduced signal detected by the detector 9, and a laser power control (LPC) circuit 11 detecting an amount of control for controlling the laser power.
The APC circuit 1 includes a detector 2 monitoring the irradiating light of laser 7, an amplifier 3 amplifying the monitored level by the detector 2, an adder 4 adding the monitored level amplified by the amplifier 3 and input to its adding input terminal (+) and an operating signal detected by the LPC circuit 11 and input to its subtracting input terminal (-).
The LPC circuit 11 includes an attenuator (ATT) 12 attenuating a reproduced RF signal SI amplified by the RF amplifier 10 by a predetermined level, a high pass filter (HPF) 13 passing a high frequency component of the reproduced RF signal SI, a peak hold circuit 15 detecting and holding a peak value in outputs of the ATT 12 and HPF 13, an adder 20 adding an output of the peak hold circuit 15 input to its subtracting input terminal (-) and a target level A input to its adding input terminal (+) for outputting an operating signal, and an amplifier 5 amplifying an output of the adder 2.
The thus configured laser power control circuit of the conventional optical device operates as follows. The reproduced RF signal SI detected by the detector 9 is supplied to the RF amplifier 10. The reproduced RF signal SI is amplified by the RF amplifier 10. The amplified RF signal SI is supplied to the ATT 12 and the HPF 13 of LPC circuit 11.
The RF signal S1 is attenuated by the predetermined level in the ATT 12 of LPC circuit 11. A DC (direct current component) of the RF signal is cut off to pass its high frequency component by the HPF 13. The attenuated output of ATT 12 and a high frequency output RFAC (S2) of HPF 13 are supplied to the peak hold circuit 15. The peak hold circuit 15 holds a peak value of an added version of the attenuated output ATT 12 and the high frequency output of HPF 13 for outputting. The peak output of peak hold circuit 15 is supplied to the subtracting input terminal (-) of the adder 20. The target level value A is supplied to the adding input terminal (+) of the adder 20. The adder 20 compares the peak output with the target level value A to output a difference between them as the operating signal which is amplified by the amplifier 5.
The operating signal is supplied to the subtracting input terminal (-) of the adder 4 of APC circuit 1. The irradiating light of laser is incident on the detector 2 of APC circuit 1 and the detector 2 monitors the irradiating light of laser 7. A voltage which is monitored by the detector 2 is supplied to the amplifier 3 to be amplified. The monitored voltage amplified by the amplifier 3 is supplied to the adding input terminal (+) of the adder 4. The adder 4 compares the operating signal detected by the LPC circuit 11 with the monitored voltage to output its difference (an amount of operation). A controlling output of the APC circuit 1 is supplied to the invertor 6 where it is inverted and supplied to the laser 7. The laser 7 emits the laser light based on the inverted controlling output. The laser light irradiates the surface of disk 8 and the detector 9 detects the reflected light, thereby allowing an information signal to be reproduced.
In this case, the peak value of the high frequency output RFAC (S2) of HPF 13 which passes the RF signal S1 from the RF amplifier 10 and cuts off the DC (direct current component) of RF signal S1 is the controlling signal, which is peak-held. This peak value of RFAC (S2) is compared with the level A (1/2.times. (the target value of amplitude value of the RF signal)) and its difference is impressed on the APC circuit 1 to control the laser power.
This makes the level of laser power to be restricted to an RF signal level established by the target level A. Here, the peak-held attenuated output of ATT 12 is used for detecting the amount of control. This is on purpose that even if the information signal is reproduced from the disk 8, e.g., with very low degree of modulation, the level of RF signal S1 can be ensured by detecting the operating signal from the LPC circuit 11. In other words, in case of the disk with very low degree of modulation, the amplitude of RF signal is small so that LPC circuit 11 operates to raise the laser power. Thus, a chain of ATT 12 is inserted in order that the RF signal is destroyed due to that operation.
In the above described laser power control circuit of the conventional optical disk device, in order to detect the amplitude of RF signal S1, only the peak value (VP+) of the high frequency output RFAC (S2) of HPF 13 is peak held as the amount of control by the peak hold circuit 15. However, because the characteristics of degree of modulation, reflective factor, etc. are different in every disk 8, the pits on the disk 8 become slightly longer or shorter by the same amount in the longitudinal forward and backward direction and so there is an irregularity of asymmetry at every disk 8. As shown in FIG. 6, the RF signal includes signals ranging from I3 to I11 between zero level and I top, but only signals within .+-.20% from the center of amplitude can satisfy the asymmetry standard. Because there is the irregularity of asymmetry in this manner, even the high frequency output RFAC (S2) of HPF 13 which cuts off the DC (direct current component) of the RF signal S1 includes signals corresponding to signals having a DC offset as shown in FIG. 7.
Therefore, if only the peak value (VP+) of the high frequency output RFAC(S2) of HPF 13 is used as the amount of control, because the peak value is controlled to be the target value, the maximum amplitude VP-P of the constant RF signal S1 which is an original object of the laser power control circuit will not be available. For example, in case of FIG. 7, because there is a positive offset, the peak value VP+ will be greater than VP+&gt;1/2.multidot. (VP-P). Thus, the amplitude VP-P of RF signal will be controlled to be smaller than the target level.
Further, in the optical disk device, when an optical pickup is moved from the inner circumference to the outer circumference (or from the outer circumference to the inner circumference) of the disk 8, for example, during the track jump TJ or sled kick, the RF signal (S1) as shown in FIG. 8A and the high frequency output RFAC (S2) of HPF 13 as shown in FIG. 8B will occur. As described above, because the peak value of the RFAC (S2) is used as the amount of control in the laser power control, the amplitude of RF signal is controlled to be greater than the target level during the movement of pickup, so that the laser power and the RF signal level will be raised in vain. This will cause an inconvenience that a life of the laser is shortened and also another inconvenience that a deviation of the gain adjustment occurs when the automatic adjustment of the servo system such as the tracking servo or the like is performed.